Prolonged iodine clearance with a depletion regimen for thyroid carcinoma: concise communication.

Prior studies suggested iodine depletion as a means of enhancing radioiodine uptake into metastases from well-differentiated thyroid cancer. Data are not available regarding the secondary effects of this maneuver on iodine clearance and on total-body radiation. Accordingly we have measured inorganic iodine clearance, total body iodine, tumor and total-body retention of radioactive iodine before and during a diet-and-diuretic regimen designed to induce iodine depletion. Total-body iodine decreased by 25% to 66% (p less than 0.05). The amount of radioiodine taken up and retained by tumor tissue increased 146% (range 48-243%) following the depletion regimen in three studies (two patients). However, because the iodine clearance decreased by 56% (range 40-71%), the total-body radiation per standard 150-mCi dose increased by 68% (range 19-111%). Consequently, the increase in lesion irradiation relative to the increment delivered to the total body was only 46% (range 24-82%). We conclude that iodine-depletion regimens are less effective than prior studies suggest.